1. Field of the Invention
This invention relates to an optical scanner to be used for an image-forming apparatus such as laser beam printer or laser facsimile machine.
2. Related Background Art
Optical scanners to be used for image-forming apparatus are adapted to reflect and deflect a light beam such as a laser beam by means of a rotary polygon mirror rotating at high speed so as to make a scanning light beam. The obtained scanning light beam is focussed on a photosensitive member arranged on a rotary drum to form an electrostatic latent image there. Then, the electrostatic latent image is turned into a visible toner image by means of a developing machine, which toner image is then transferred onto a recording medium such as a sheet of recording paper. Thereafter, the toner on the recording medium is heated and fixed to complete the printing process.
FIG. 1 of the accompanying drawings schematically illustrates a known typical optical scanner Eo. Referring to FIG. 1, the laser beam (light beam) emitted from a semiconductor laser 101 is collimated in a lens barrel 100 and then converged to a linear light beam by means of a cylindrical lens 130. Then, the light beam is deflected by a rotary polygon mirror 102 to scan in a predetermined direction (main-scanning direction) that is perpendicular to the axis of rotation of the polygon mirror and subsequently focussed on a photosensitive member 215 arranged on a rotary drum by means of imaging lenses 103a and 103b. As the light beam striking the photosensitive member 215 is made to scan in the main-scanning direction by the rotation of the rotary polygon mirror 102 and also in the sub-scanning direction by the rotation of the rotary drum, it forms an electrostatic latent image on the photosensitive member.
As the above-described scanning operation for writing image information on the photosensitive member 215 of the rotary drum is repeated, there can arise a problem that the starting point of the writing cycle may be displaced form cycle to cycle due to possible division errors of the reflecting planes of the rotary polygon mirror 102. To avoid this problem, the scanning light beam coming form the rotary polygon mirror 102 is reflected by a BD mirror and led into a BD sensor when the bet gets to the end of the plane being scanned. Then, the controller of the BD sensor transforms the introduced light beam into a scan start signal. The semiconductor laser 101 is so arranged that it starts another write modulation cycle upon receiving the scan start signal.
The imaging lenses 103a and 103b are a spherical lens and a toric lens and have a so-called fxcex8 function of transforming the scanning light beam made to move at a constant angular velocity by the rotary polygon mirror 102 into a scanning light beam moving at a constant velocity in the main-scanning direction on the rotary drum. The rotary polygon mirror 102, the motor for driving the polygon mirror 102, the imaging lenses 103a and 103b are contained in an optical cabinet 110 whose top opening is hermetically sealed by means of a lid member 120 having a radiator panel 121. In FIG. 1, the radiator panel 121 and the lid member 120 are indicated by broken lines to show their positions.
As a result of the technological development in recent years, image-forming apparatus are made to operate at high speed to produce high density images. However, this technological trend requires the rotary polygon mirror to operate with a large number of revolutions per unit time.
On the other hand, as the rotary polygon mirror is driven to rotate at high speed, there arises problems including those of vibrations, noises and heat generated particularly by the bearing and the motor of the rotary polygon mirror. The noises generated by the rotary polygon mirror and the motor can end up with abnormal vibratory sounds. Some known optical scanners are provided in the inside thereof with a noise absorber to minimize the noise problem. The heat generated by the motor can by turn significantly raise the temperature of the optical cabinet to consequently degrade the performance of the motor and that of the optical components housed in the optical cabinet. Although the heat problem may be partly dissolved by arranging radiator fins and/or a heat exchanger in the inside, such an arrangement can greatly increase the dimensions of the apparatus and raise the assembling cost.
An effective way of alleviating the noise problem and the vibration problem of the rotary polygon mirror is to reduce the number of revolutions per unit time of the rotary polygon mirror. Meanwhile, so-called multi-beam deflection scanners adapted to use a plurality of light sources and hence so many light beams simultaneously are known. Such scanners are designed to produce high density images at high speed if the rotary polygon mirror is driven with a relatively small number of revolutions per unit time. If, for instance, two light beams are used, the photosensitive member of the rotary drum is exposed to two light beams simultaneously. Therefore, the time required for the photosensitive member to be fully exposed to light can be reduced to a half of the time required for the exposure process of the photosensitive member using a single light beam. Similarly, if four light beams are used, the number of revolutions per unit time of the rotary polygon mirror can be reduced to a quarter of that of the rotary polygon mirror using a single light beam.
Thus, the use of a plurality of light sources is effective to avoid the problems arising form the-high speed operation of the rotary polygon mirror and including those of vibrations, noise and heat. However, with the known technology, a plurality of light sources are arranged independently in the optical cabinet so that each of them requires a cumbersome operation of aligning the optical axis and securing it to the cabinet by means of screws. Therefore, the use of a plurality of light sources inevitably entails an increased number of parts to be assembled. Additionally, each of the light sources requires a considerable space for accommodating the screws securing it to the cabinet to make the latter dimensionally remarkable.
Then, the light beams emitted from the plurality of light sources are deflected by the rotary polygon mirror to scan the photosensitive member in the main-scanning direction with regular intervals separating them. As a result, the photosensitive member is exposed to light.
In order for any adjacently located ones of the plurality of light beams to be separated from each other accurately by a predetermined distance, all the light sources have to be accurately positioned in and secured to the optical cabinet. Then, positioning the light source accurately is a painstaking operation and requires a large optical cabinet.
More specifically, in order to realize a pixel density of 600 dpi, for instance, the intervals separating the scanning lines on the photosensitive member have to be regulated so as to be equal to 42.3 xcexcm. Then, the operation of aligning the optical axis of each of the light sources is time consuming. Thus, there is a demand for an optical scanner that provides an improved efficiency for assembling.
In view of the above identified problems of the prior art, it is therefore an object of the present invention to provide an optical scanner designed to comprise a plurality of light sources for the purpose of high speed and high density printing that allows a simplified operation for securing the light sources in position and requires only a limited space for the light sources to reduce the size of the optical cabinet and the cost of assembling it.
Another object of the present invention is to provide an optical scanner comprising a plurality of light sources that are unitized with respective collimator lenses to allow a simplified operation for securing the light source units to the optical cabinet and the use of only a limited space to accommodate the light source units.
According to the invention, the above objects and other objects are achieved by providing an optical scanner comprising a plurality of light sources, a plurality of light source holders for securing the light sources, a deflection/scan means for deflecting the light beams emitted from the light sources and causing them to scan, a cabinet for accommodating the light sources and the deflection/scan means and an imaging optical system for focussing the light beams coming from the deflection/scan means on an imaging plane, one or more than one light source holders being adapted to secure two light sources.
In another aspect of the invention, there is also provided an optical scanner comprising a light source device having a plurality of light source units, a deflection/scan means for deflecting the light beams emitted from the light source units and causing them to scan, a cabinet for supporting the light source device and the deflection/scan means and an imaging optical system for focussing the light beams coming from the deflection/scan means on an imaging plane, the light source device having a unitizing means for unitizing every two of the plurality of light source units as sub-units and a securing means for securing the sub-units to the cabinet independently.
Preferably, the securing means has a rotary angle regulating means for rotating each of the sub-units relative to the cabinet.
Preferably, each of the sub-units is made rotatable around one of the paired light source units.
Preferably, the securing means has a position regulating means for regulating the intervals separating the sub-units.
Preferably, the light source device has a light path regulating means for making the angles of incidence of all the light beams equal relative to the deflection/scan means.
If each light source unit adapted to emit a light beam is secured to an optical cabinet independently, the cabinet requires a large space for securing all the light source units on a one by one basis. According to the invention, every two of the light source units are paired and unitized as sub-unit to produce two or more than two sub-units, each being adapted to emit two light beams, and the sub-units are secured to the cabinet to reduce the amount of the overall securing operation and also the space required for securing them to the cabinet. Additionally, the intervals separating the optical axes of the light beams and hence the scanning lines of the light beams on the imaging plane can be regulated by regulating the rotary angle of each of the sub-units independently and/or by regulating the intervals of the sub-units. As a result, all the intervals separating the scanning lines can be made to accurately agree with the designed value. The light sources, not unitized into light source units, may also be arranged in a similar manner to achieve the same result.